As smartphones with rollable screens, smartwatches that monitor ECGs, and AR glasses that fit seamlessly on the face become more common, flexible electronics are transitioning from concept to reality. In this wave of human-machine integration, devices not only need bendable screens and circuits but also require materials that can safely interact with skin, absorb impacts, provide sealing protection, and remain optically transparent—acting as "soft interfaces" for human-machine interaction. Among these materials, silicone rubber, especially high-transparency liquid silicone rubber (LSR), stands out due to its unique combination of properties, making it essential for flexible display backplanes, wearable sensor encapsulation, and device components.

1. Optical Transparency and Flexible Support

Flexible OLED or Micro-LED displays can be bent, but their ultra-thin glass or polyimide (PI) substrates are still susceptible to scratches and impacts. To address this, manufacturers often cover the back or edges of these displays with a layer of highly transparent silicone rubber film (transmittance >92%):

Impact resistance: Provides cushioning to prevent shattering upon drops.

Surface treatments: Can be coated with anti-fingerprint (AF) or anti-reflective (AR) coatings.

Bendability: Maintains optical clarity even at bending radii <5 mm without whitening or stress marks—superior to TPU or PET.

For example, some foldable phones use transparent silicone gel in the hinge area to conceal mechanical structures while providing a continuous visual experience.

2. "Electronic Skin" for Wearable Sensors

Smart bands and health patches need to adhere to the skin for extended periods to collect physiological signals such as ECG, PPG, and EMG. Here, silicone rubber plays three critical roles:

Biocompatible substrate: Certified under ISO 10993, non-allergenic, suitable for sensitive skin.

Elastic encapsulation: Encases flexible circuits and chips, offering water and sweat resistance (IP68) while allowing stretching.

Functional integration platform: Incorporates conductive fillers like liquid metals to create strain sensors or embeds microfluidic channels for sweat analysis.

Apple's Apple Watch, for instance, uses medical-grade silicone gaskets around the heart rate sensor to ensure tight contact between the optical window and the skin, reducing motion artifacts.

3. Sealing and Adhesion: Reliable Miniaturization

Space inside wearables is limited, making traditional screws and adhesive strips impractical. Silicone rubber can be applied via selective dispensing or in-mold decoration (IMD) to achieve:

Acoustic sealing for miniature microphones/speakers.

Waterproof barriers around battery compartments and charging contacts.

Stress-relieving adhesion between flexible FPC cables and motherboards.

Its low elastic modulus (0.5–2 MPa) absorbs repeated deformations during daily activities, preventing solder joint fatigue and failure.

4. Feel and Aesthetic Design

Consumer expectations for the "feel" of wearable devices are becoming increasingly sophisticated. Silicone rubber can be tuned to mimic the softness of human skin (Shore hardness 30A–50A) and supports:

Various surface finishes including matte, glossy, and textured surfaces.

Multi-color injection molding to create gradients or functional zones (e.g., non-slip areas on fitness trackers).

Seamless coating over metal or ceramic materials, enhancing premium aesthetics.

5. Challenges and Future Directions

Improved stain resistance: Development of oil- and water-repellent surfaces to minimize sweat residue.

Self-healing capabilities: Introduction of dynamic bonding networks allowing heat-induced scratch repair.

Biological integration: Combining with live cells or enzymes to achieve closed-loop biochemical sensing.

Conclusion

In the realm of flexible electronics, silicone rubber acts as the silent "tactile translator"—transforming cold circuits into comfortable wearables and converting precise optical signals into reliable health data. Though it does not emit light, it makes screens more durable; though it does not compute, it ensures sensor accuracy. This transparent, soft interface adds a layer of reassurance to every human-machine interaction, bridging the gap between people and technology. It is this subtle yet crucial material that makes our connection with machines safer, more intuitive, and ultimately, more human.


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